Technical Field
The present invention relates to an electromagnetic flowmeter, and particularly, to a technology of detecting an abnormality based on characteristics of a fluid to be measured.
Related Art
Since an electromagnetic flowmeter configured to measure a flow rate of a conductive fluid by using electromagnetic induction is robust and highly precise, it is widely used for industrial utilities. The electromagnetic flowmeter is configured to enable a conductive fluid to be measured to flow into a measurement tube to which a magnetic field is applied in an orthogonal direction, and to measure a generated electromotive force. Since the electromotive force is proportional to a flow velocity of the fluid to be measured, it is possible to obtain a volumetric flow rate of the fluid to be measured on the basis of the measured electromotive force.
The generated electromotive force can be measured by a pair of electrodes (an electrode A503a and an electrode B503b) attached to a measurement tube 501, as shown in FIG. 14, for example. Also, the magnetic field in the orthogonal direction can be generated by enabling excitation current to flow from an excitation circuit 505 to an excitation coil 502 arranged in the vicinity of the measurement tube 501.
In general, as the excitation current Iex, an alternating current of which a positive excitation period and a negative excitation period are switched with each other is used. However, Patent Document 1 discloses a technology of enabling the excitation current Iex of which a non-excitation period is provided between the positive excitation period and the negative excitation period to flow to the excitation coil 502.
Regarding the electromagnetic flowmeter in which the non-excitation period is provided between the positive excitation period and the negative excitation period, Patent Document 2 discloses a configuration where upon switching to the non-excitation period, differential noises generated at the electrode A503a and the electrode B503b are summed and an abnormality, which occurs when the measurement tube 501 is empty or insulating foreign matters are attached to the electrode A503a and the electrode B503b, is detected on the basis of the summed value, as shown in FIG. 15.
According to the technology disclosed in Patent Document 2, since the differential noise is used as an abnormality determination standard, voltages generated at both the electrodes are acquired for a period T1, a period T2, . . . within predetermined time after the switching to the non-excitation period.    [Patent Document 1] Japanese Patent Application Publication No. Hei 3-144314A    [Patent Document 2] Japanese Patent Application Publication No. 2011-209231A
The abnormality that occurs in the electromagnetic flowmeter may also occur in other cases as well as when the measurement tube is empty or the insulating foreign matters are attached to the electrodes. For example, an abnormality based on characteristics of the fluid to be measured (process fluid) may occur.
As the abnormality based on characteristics of the fluid to be measured, a case where air bubbles are generated in the fluid to be measured, a case where the fluid to be measured becomes a low conductive fluid, a slurry fluid and the like may be exemplified. Also, a case where an electrode made of a material corresponding to the fluid to be measured is not selected and electrode corrosion due to the acidic/alkali fluid to be measured are also caused due to the abnormality based on characteristics of the fluid to be measured. Also, the attachment of the insulating foreign matters to the electrodes may be included in the abnormality based on characteristics of the fluid to be measured.
It cannot be said that the abnormality based on characteristics of the fluid to be measured always comes to the fore as a magnitude of the differential noise. Therefore, the abnormality determination method using the differential noise, which is disclosed in Patent Document 2, is not appropriate for the abnormality detection based on characteristics of the fluid to be measured. For this reason, it is needed a technology capable of simply detecting the abnormality based on characteristics of the fluid to be measured.